While much is known about the organization of the peripheral and central connections that subserve the sense of touch, little is known about how these precise connections develop. The proposed experiments will investigate the development of the most peripheral of neuronal contacts in the somatosensory system, the contacts that cutaneous sensory afferents establish in the skin. Cutaneous axons from each dorsal root ganglion (DRG) innervate a characteristic region of skin, referred to as a dermatome. The overall objective of the proposed experiments is to determine whether DRG neurons are specified to innervate their characteristic region of skin prior to axonal outgrowth, and whether these neurons are directed to their usual skin region by cues in the surrounding environment. As a first approach to this problem, the dermatome of each DRG supplying the hindlimb will be mapped in hatched chicks and at selected embryonic stages to determine the sequence of events during the normal development of dermatomes. The mapping will involve extracellular electrophysiological recording from each dorsal root, and light and electron microscopic examination of cutaneous sensory nerve endings that have been labeled with horseradish peroxidase or a radioactive precursor. In later investigations surgical manipulations involving skin transplantations, DRG deletions, and limb bud and spinal cord rotations will be made in embryos, and the resulting dermatomes mapped. The establishment of correct innervation by the DRGs in operated embryos will be taken as evidence that the cutaneous afferents are prespecified, and that the specification plays a role in the normal development of dermatomes. The failure of DRG neurons to innervate their correct skin region would suggest that during normal development the axons are directed to their characteristic region of skin by cues in the limb environment.